In antilock braking it is important to have good vehicle speed information as a reference against which wheel speeds during braking can be compared whereby the amount of wheel slip is determined. While sensors on non-driven wheels of a vehicle provide such information when brakes are not applied, such information does not represent vehicle speed when brakes are applied. Elegant methods such as radar speed sensing have been proposed but the relatively high expense prohibits its use. Accordingly, practical antilock systems rely on inexpensive vehicle speed estimation techniques.
It is well known to use a chassis accelerometer to determine vehicle acceleration and integrate its output to get a measure of vehicle speed but the accelerometer is subject to drift as well as to the gravitational effect when the vehicle is on a slope. Thus it is also well known that it is very difficult but crucial to estimate the accelerometer reference level or bias prior to braking so that the integration can remain stable when the vehicle is braking on a slope.